This invention generally relates to electrical signal transmission and more particularly to conductors that may be applied to the transmission of power and/or information signals from one member rotating with respect to another member.
In the field of monitoring for a condition occurrence on a first member rotating with respect to a second member, the motion interface between the two members has always posed a particular signal transmission problem. Solution of the problem in some applications involved elaborate mechanical configurations of carbon brushes contacting metal rings and in other applications involved electrical coupling techniques including inductive, capacitive and/or impedance coupling configurations. While these techniques have met with success in various limited applications they have not been entirely successful in vehicle applications wherein extremes of pressure, temperature, and/or corrosion are experienced. For example, in an aircraft type application wherein it is desired to transmit low power signals indicative of a particular monitored condition on a wheel to a signal detector mounted on the landing gear strut, the motion interface between the wheel and axle poses a particular difficult signal transmission environment. While various type low noise slip rings are known and used in the electronics arts, their use in an aircraft type application and especially as it pertains to the transmission of signals across a wheel/axle interface is prohibited by reason of the following considerations:
(a) The lifetime of a precision low noise slip ring is directly proportional to its diameter. For example, a 0.7 mm shaft would have a 50,000 mile running lifetime (approx. 10 yrs) while a slightly larger 10 mm shaft would be limited to 5,000 mile running lifetime (approx. 1 yr). In the aircraft application of interest the shafts must be capable of load bearing and therefore of necessity are of the larger diameter and in the range of the low lifetime slip ring. A 5,000 mile one year lifetime cannot be tolerated in the aircraft application. PA1 (b) The standard slip rings used in low noise signal processing applications are vulnerable to a plating type failure mechanism known as polymer buildup. The hydraulic fluids and bearing lubricants commonly found in the aircraft type application of interest adversely react with the precious metal platings on the conventional low noise slip rings to produce polymer films which lead to short term wear and other failure mechanisms. PA1 (c) State of the art slip rings used to send low level signals exhibit optimum operation when rotating, and are subject to oxidation and insulation problems when stationary. The high accelerations in an aircraft application at touchdown adversely affect such standard slip rings. This acceleration problem is accentuated when the slip rings are also subjected to hostile temperature, humidity and the contaminated environment of the aircraft wheel assembly. The normal aircraft wheel sees impact forces to 100 G's vertical and .+-.15 G's side loading with rotational forces resulting from accelerations to 250 mph in one quarter rotation. A slip ring bounce or breakaway would create an unacceptable error signal.
It is the purpose of this invention therefore, to provide a rotating type conductor that is not limited by shaft diameter and is substantially insensitive to environmental conditions while providing a low noise transmission path that is effective for low power signal detection between the two relatively moving members.